In China’s northern dialects, the number two, 2 (pronounced “er” in Chinese), means stupid or retarded if used as an adjective. For example, he is so 2 = he is so stupid. Similarly, you can say “Her action is very 2″, “The show is 2″, etc.
Since today is Tuesday, Feb, 22nd in China, literally 2,22,2, four 2s. Chinese netizens call today the Day of 2 and are ready for a celebration. How? That depends. All these young Chinese need is another excuse to let loose and have fun.